Guided wave optical devices, such as switches and filters, tend to operate differently for different directions of wave polarization. This creates a problem inasmuch as available single mode fibers do not preserve any particular direction of polarization. As a result, a linearly polarized optical signal applied at the input end of a fiber emerges at the output end with an arbitrary elliptical polarization that can vary with time. Under these circumstances, a single polarization switch or filter would yield unacceptably high crosstalk and loss whenever the polarization of the received signal is different than the particular polarization for which the switch is designed.
Efforts to obviate this problem have been directed to both the fiber and the devices. With respect to the former, specially fabricated birefringent fibers, that maintain linear polarization, are currently under investigation and have been demonstrated for only short lengths. In addition, questions concerning loss, cabling and splicing have not been addressed.
With respect to the devices, filters suitable for single mode multiplexing/demultiplexing include planar corrugated waveguides (gratings), and an electrically tunable directional coupler. Such devices, when fabricated in birefringent substrates result in a filter response whose center wavelength depends upon polarization. The filter center wavelength for grating filters is directly proportional to the effective index seen by the respective modes. For the directional coupler filter, the phase match wavelength depends upon the intersection of the dispersion curves for the two waveguides. In either case, material and modal birefringence wil normally result in different center wavelengths for the TE and TM modes.
A further difficulty relates to tuning, and is due to the fact that the orthogonally polarized modes see different electrooptic coefficients. As a result, changes in the two indices, produced by a common tuning voltage, are different. This makes it impossible to simultaneously tune the two polarizations to the same filter center wavelength by means of a single tuning signal.
In U.S. Pat. No. 4,273,411, a wavelength filter is described which, as a practical matter, can be made to be polarization independent. However, such a device is not inherently polarization independent in that the effective refractive indices for the TE and TM modes are not necessarily equal.